“The works in this exhibition celebrate the last half century of bridge projects and the engineering that has made them possible. They also show how the use of film and photography has changed since the first decades of Ove Arup and Partners – founded in 1946.”
In this guest posting Joseph describes the story behind the thermal imaging.
Millennium Bridge Story
Cold blue and hot red: can thermal photography help the Millennium Bridge to reveal itself? Does enhanced perception tell us a different story from the obvious, the everyday, the one which we already know?
A thermal journey across the Millennium Bridge reveals a strange new perceptual stage in which the “things in themselves” occupy unexpected places and exhibit unexpected shapes and colours. Parts of the bridge, parts of the city and parts of the people suddenly appear strange and unexpected. The metal supports of the bridge cool in the wind while the solid masonry foundations stubbornly retain their heat. People appear as bright glowing light bulbs, centres of heat, moving over and around the bridge in their living, unmechanical, way. Interaction occurs, with people imprinting their life force on the bridge through heat transfer from direct contact. The dome of St. Paul’s cathedral glows red as its lead covering heats in the afternoon sunlight. London’s masonry and glass glow.
To thermal eyes the Millennium Bridge reveals a new version of its story. This exhibition provides many views, and thus many stories, which are told through thermal photography. Often seen as technical tools, thermal imaging cameras can also act as translators between ourselves and our physical world, expressing sensations which cannot be stated in words, and capturing photographic insights which are lost in the visual spectrum due to clutter, confusion and overwhelming detail.
Thermal photography helps to reveal a secret life of Millennium Bridge, that of heat and energy. The choreography of sun, wind, materials, physics and living creatures is revealed to thermal eyes, and the many secret stories of everyday bridge life are told from a different perspective and a different point of view. Stability and motion, man and nature, routines of everyday life, all these plots and more are acted on the thermal stage which is Millennium Bridge.
Guide to the Thermal Images
The thermal images of this exhibition all 320X240 pixel JPEG images shot using a 60 Hz thermal imaging camera which was similar in appearance to a camcorder. Since such cameras measure a property, temperature, which is not part of the visible light spectrum, pseudo-colour was used to indicate the variations in temperature. The pseudo-colour scheme adopted was bright red-orange for the hottest temperature found in the individual image while dark blue was used for the coolest. Since the pseudo-colour scheme was normalised for each image individually, the same colour can indicate different temperatures when appearing in different images. For the current exhibition, therefore, colour should be considered to provide a measure of relative temperature rather than of absolute temperature.
Thermal imaging, or, more precisely, infrared thermography, consists of measuring the infrared radiation of the electromagnetic spectrum from approximately 900 to 14,000 nanometres of wavelength. Infrared radiation is one region of the electromagnetic spectrum, other regions being for example those of the gamma rays, x-rays, ultra violet light, visible light and radio waves. Infrared radiation is emitted by all objects and the amount of emitted radiation increases with increases in the temperature of the object. The temperatures which can be measured by means of a modern thermal imaging camera are normally from approximately -50 °C to 2,000 °C.
Infrared radiation is measured using a thermal camera in much the same way that visible light is measured using a digital camera. However, while digital cameras use a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS) sensor, thermal imaging is based on the use of focal plane array (FPA) sensors which respond to the longer wavelengths of the infrared region of the electromagnetic radiation. Given the complexity of the FPA sensors, the maximum resolution which can currently be achieved is lower than that of CCD or CMOS sensors. Most thermal imagining cameras have the relatively low resolutions of 160×120 pixels or 320×240 pixels, with the most expensive current models reaching 640×512 pixels.
While the amount of thermal radiation depends greatly on the surface temperature of the object which is being measured, the surface temperature is not the only factor involved. Other factors which effect the measurement include the emissivity of the object which is being captured, the amount of radiation arriving from the surrounding environment and the atmospheric absorption between the radiating object and the thermal imaging camera. Emissivity and atmospheric absorption thus affect the measured temperatures, and if not carefully compensated at the time of each measurement can lower the accuracy of the temperature values.
Of the factors effecting the accuracy of thermal images, the biggest is the emissivity, meaning the ability of the object’s material to emit thermal radiation. Every material has an emissivity value which is in the range from 0.0 (no ability to emit thermal energy) to 1.0 (complete emission of all thermal energy). In addition, the emissivity value is not a fixed value for most materials, but is actually a continuous function of the temperature. Given the complex physics, the maximum theoretical measurement accuracy of a thermal camera is achieved only when the emissivity value of the object which is being studied is known or when the camera can be calibrated on-sight against a known reference source of thermal radiation. In the case of the images found in this exhibition, the camera was set to run using a stored internal emissivity table, thus the camera was not calibrated for each shot so as to achieve the maximum possible accuracy.
[If you like these images, you may also be interested in his new book “Seeing the World Through 21st Century Eyes“]
And finally, it’s almost 10 years old, but still fascinating to watch, a video shot during testing of the Millennium Bridge where the footfall of 2000 members of the public was being monitored against the Synchronous Lateral Excitation of the bridge.
And one from the local news on the opening day…